And it's because, with luck being equal, he could beat anyone else. Rather straightforward definition of best.

"Hey you, bats should be nerfed."
"Why?"
"Because I lost a game to bat swarm and I'm bitterUhm... clarity... and... consistency? Yeah yeah that sounds good. Clarity and consistency."
Do not. Nerf. The bat.

However that doesn't really answer the "best Wesnoth player ever" question. For that you might look at some of the past tournaments and see who consistently ranked near the top. Incidentally, I noticed that Kira1 who is currently #1 on that ladder, was also the winner of the Wesnoth Time Twister v1.9 tournament. It would be cool if someone could put a list together of all the past tournaments/competitions and their winners.

Keep in mind that being the best in MP does not mean that one is also the best in SP. Even for MP, there are different modes of play, e.g. speed play, 2v2, PickYourRecruits, etc. So it might help to be more specific with your question and your purpose for asking.

PickYourRecruits? I would assume the standard for best at Wesnoth would be default era, and multiplayer. Timed or not. Also, is it really impossible to prevent cheating as of now? I heard that people who rely on cheating are eventually found out, and cheating doesn't makes much of a difference, but what if someone good who doesn't usually cheat, decides to get a little help in only one match?

As for the ranking, I've noticed that not everyone is on it (like ShadowVeil who seems to be pretty good).

Purpose? Curiosity, really.
I had already googled for it and was a bit surprised this wasn't discussed here before. Apparently people don't really care!

As I said there is no defenite best player, the reasons are many and complicated, there are good and bad players, but there is no certain way to asset who is the best player ever, why?
Well first thing is that luck is also involved, with 2 very good players, luck can shift the ballance.
Then let's assume that currently there is a very skilled player and some ppl say that he is the best, but that does not mean that he is the best one ever, because how you know if the top player that used to play like 3, 5, 10 or more years ago was better or worse if he and the actual one never played one against other,( could be for various reasons like one quit playing years ago snd so on), and even if they did, just one match would be not enought to judge, also cause of luck and other reasons, like one was very tired or sick that day, so his performance was worser.
For istance one of my friends managed to beat a player that was at the first place in ladder rankink, and he is not even in the first 10 ir 20.
What that means, he is better or worser ?
I saw some player loosing a lot against players that were considered "worser" but rhen winning multiple times again other players considered "better.
Another evidence that there is no best player (or anyway no way to tell) is that ppl don't agree all on who is the best one, this was actually discussed, kinda, someone asked who are the best Wesnoth players, and ppl came up with different names, but its not that they all agreed on who was the best one.
As for ShadowVeil, it came out that he was suposely a cheater, you can still read the article on the ladder website and he was in fact banned.
So we have cheating too, who knows for sure who cheats and who doesn't ?
Or who does only sometime?
The factors are simply to many to assert for sure, you can just make guesses based on personal preferences or bits of data gathered arround.

Just wondering, but wouldn't the best be someone who understands more the game (has more knowledge), and adapts better than others to every luck outcome and new information available each turn, or during the turn? Who reads the game, knows the options during the game and uses the one with a higher chance to work (or one with a chance of working that he has to bet on during a given situation), and keeps adjusting and changing, always conscious of the chances. Although he might not win it all, and it might even come down to guesses that will give him that plus as a player.

Well, that's quite subjective and might be the case for every luck based turn games.

But, of course, if there's cheat involved then it's harder to judge...

Ok, I think this has gone too far XD I might have been the guy who told you Chosen that Benzi is the best as I remember some guy (maybe it was you) asking about that, and I have to say that it was just a joke xD

@Elder but we have asked for the disclosure of all the cheese and we treat it seriously. Everyone on this site can get banned, everyone.
@Elder ale prosiliśmy o wyjawianie wszystkich serów i traktujemy to poważnie. Każdy na tej stronie może zostać zbanowany, każdy.